Currently, sublimation type thermal transfer recording method is known, wherein a thermal transfer sheet in which a sublimation type dye is supported on a substrate made of a plastic film or the like, and a thermal transfer image-receiving sheet in which a receiving layer is provided on a substrate made of a paper, a plastic film or the like are superposed to form a full color image. Since this method uses the sublimation type dye as a color material, this method excels in reproducibility and gradation of halftone, and thus, a full-color image can be clearly expressed as the original image on the image-receiving sheet. Therefore, this method has been applied in the color image formation for digital cameras, video recorders, computers or the like. Its image can rival silver halide photography with high quality.
As the thermal transfer image-receiving sheet to be used for the sublimation type thermal transfer recording method, a solvent type thermal transfer image-receiving sheet that has a solvent type dyestuff receiving layer of the solvent system, and an aqueous type thermal transfer image-receiving sheet that has an aqueous dyestuff receiving layer are known in the art. When printing is performed under a high temperature and high humidity condition with using these thermal transfer image-receiving sheets, the dyestuff receiving layers thereof come to suffer easily damages since the substrate and the dyestuff receiving layer lose their stiffness by taking up moisture, in both cases of the aqueous type transfer image-receiving sheet and the solvent type transfer image-receiving sheet. And, in the case that the dyestuff receiving layer suffers damages on the printing, a problem that the gloss of the sheet becomes low arises.
We, the inventors have found that the damages suffered on the dyestuff receiving layer were mainly caused by degression of slippage between the thermal transfer sheet and a thermal head on the printing operation. Concretely, we have found that the dyestuff layer of the thermal transfer sheet suffered damages when the slippage between the thermal transfer sheet and the thermal head was low. We have also found that the damages had a bad influence on the dyestuff receiving layer, and thus the gloss of the printed matter became low.
As the thermal transfer sheet which can improve the slippage between it and the thermal head, a thermal transfer sheet which has a back face layer provided on a surface of the substrate which is opposite to the surface onto which the dyestuff layer is provided has been known. For instance, the Patent Literature 1 discloses the one of which back face layer is formed by coating a coating solution onto the substrate and drying it, wherein the coating solution is prepared by dissolving or dispersing a silicone modified urethane resin, a silicone modified acrylic resin, and a polyisocyanate curing agent in an appropriate solvent. Besides, the Patent Literature 2 discloses a thermal transfer sheet in which aback face layer having a cross-linking structure is formed on the substrate by using concurrently a binder resin which has a reacting group such as hydroxyl group with a polyisocyanate. As for the back face layers disclosed in the Patent Literatures 1 and 2, heat resistance is given on the back face layer by curing the binder resin with the curing agent. Moreover, these Patent Literatures disclose that it is preferable to contain a lubricant ingredient such as phosphoric ester, silicone oil, zinc stearyl phosphate, etc., in order to give the slipping property to the back face layer.
However, there is a problem that the gloss decreases when the printing is done under a high temperature and high humidity environment with respect to the thermal transfer sheets disclosed in the Patent Literatures 1 and 2 and the thermal transfer sheet which has a back face layer known in the art. Moreover, there is another problem of generating printing wrinkles when the printing is done under a low temperature and low humidity environment. It is necessary to improve the slipping property and the heat resistance of the back face layer in order to solve these problems. As a way for improving the slipping property, to increase the amount of the lubricant in the back face layer would be considered. However, some residues might adhere to a thermal head on the printing operation in the case that the lubricant is increased, and thus, there is a possibility of occurring a surface flaw or a hairline defect on the printed matter.
In the Patent Literature 3, a thermal transfer sheet which is provided with a back face layer which comprises a cellulose acetate butyrate (A1), at least one resin (A2) selected from the group consisting of acrylic resin and polyvinyl acetal resin, and a lubricant is disclosed.
Even in the case of the thermal transfer sheet disclosed in the Patent Literature 3, however, the dyestuff layer of the thermal transfer sheet suffers damages and the gloss of the printed matter decreases when the image is formed under a high temperature and high humidity environment. Further, the printing wrinkle is generated when the printing is performed under a low temperature and low humidity environment.